random ping timeouts btw. WAG54G & WRT54G (~10%)

I got a new ISP and therefore the Alcatel Speed Touch was replaced by a WAG54G.
On WAN the WAG54G is connected to the internet and on the LAN port the WRT54G (V2) is connected (non CAT5 cable). I know that's not the best way but I cannot change the cable anymore. It's in the wall and can not be replaced.

Before the WRT54G I used a SMC7004BR with the Alcatel Speed Touch and everything was fine. No time outs on the ports.
Using the WAG54G with the SMC7004BR behind also no time outs.
Using the WAG54G with the WRT54G there are unregular time outs. About 10 %. It does not depend if I send 100 or 1000 pings.

If I use the old SMC device instead of the WRT54G I do not have problems. I guess the problem is the bad cable and the SMC is more resistant on the interference.
Maybe someone knows if I can manually set the port speed to 10 M instead of AUTO. Maybe this would reduce problems.
Changes on the firmware did not help. Also changes on the MTU did not help.
FW Version WAG54G: WAG54GV2-EU_V1.00.19
FW Versions of the WRT54G: HyperWRT_2.0b4_G.bin, I tried also HyperWRT_2.1b1_G.bin -> no sucess

I tried it also with the original Linksys FW WRT54GV2_3.03.1_ETSI_code.bin which did not help as well.

I also searched the net for shell commands to find some settings directy on the shell but without success.

If I use a short cable (2m) delivered with the routers and connect directly no problem.

Maybe someone of you knows how to handle this problem and please do not suggest to replace the cable. As already described: not possible

Thanks for your answer. Thought that there might be a hidden configuration mode on command shell to adjust collisions or delay or something like that because with the old SMC7004BR I have no time outs.

I do not need 10/100 MB between the two Linksys. If the bandwith is 2 MB it's more than enought.